Shunt systems for directing body fluid from one region to another are known in the medical field. One application for such a shunt system is in the treatment of hydrocephalus, a condition where cerebrospinal fluid (CSF) collects in the ventricles of the brain of a patient. CSF is produced by the ventricular system and is normally absorbed by the venous system. However, if the CSF is not absorbed, the volume of CSF increases thereby elevating the patient's intracranial pressure. This excess CSF can result in abnormally high epidural and intradural pressures. Left untreated, hydrocephalus can result in serious medical conditions, including subdural hematoma, compression of the brain tissue, and impaired blood flow.
Shunt systems have been developed to remove the excess CSF and to discharge the fluid to another part of the body, such as the peritoneal region. By draining the excess fluid, the elevated intracranial pressure is relieved. FIG. 1 illustrates an exemplary prior art shunt system 10 having a ventricular catheter 12 inserted through a hole 14 in the skull of a patient. The catheter 12 is advanced through brain tissue 16, and into a ventricle 18 of the brain where excess CSF is present. The catheter 12 is coupled to an inlet end of a shunt valve 20 and a drainage catheter 22 is coupled to an outlet end of the shunt valve. The shunt valve 20 is typically implanted under the scalp (not shown) of the patient. The shunt system is operative to drain excess CSF fluid from the ventricle to another part of the body, such as the right atrium, peritoneal cavity, or other locations in the body.
Generally, fluid shunt systems include a valve mechanism for controlling or regulating the flow rate of fluid through the system. Illustrative valve mechanisms operate to permit fluid flow only once the fluid pressure reaches a certain threshold level and may often permit adjustment of the pressure level at which fluid flow commences. The fluid flow rate is proportional to the pressure difference at the valve mechanism. Thus, for a pressure slightly greater than the threshold pressure level, the flow rate is relatively low. As the pressure increases the flow rate through the shunt system concomitantly increases. With these shunt systems, fluid flow normally continues until the intracranial pressure has been reduced to a level less than the threshold pressure, subject to any hysteresis of the device.
Effective fluid flow rate control is particularly important in these kinds of shunt systems since overdrainage of CSF can result in dangerous conditions, including subdural hematoma. Overdrainage tends to occur when a patient moves from a horizontal position to a sitting or standing position, due to a siphoning effect in the shunt system. That is, when the patient is lying down, the ventricle, which contains the proximal end of the shunt, is at the same elevation as the abdomen, which contains the distal end of the shunt. CSF flows out of the head normally when the pressure differential between the ventricle and the abdomen exceeds the setting of the pressure valve. However, when the patient rises to a standing or sitting position, the elevation of his head with respect to his abdomen increases. The siphoning effect is a result of the increase in the pressure differential due to gravitational effects resulting from the increased vertical height of the fluid column between the patient's head and the selected drainage location elsewhere in the patient's body. Thus, the valve may open and allow flow even though other conditions have not changed. Although such an increase in differential pressure is normal, in typical shunt systems the opening of the valve will result in undesired overdrainage, or siphoning, of the ventricular spaces.
Anti-siphon shunt systems have recently been developed which minimize the occurrence of overdrainage in patients. These anti-siphon shunt systems generally provide valve mechanisms which open, or drain, only when the intracranial pressure, or proximal pressure at the upstream side of the valve, rises above a predefined threshold pressure established in relation to some fixed reference pressure such as subcutaneous pressure. Along with the subcutaneous pressure, this predefined threshold pressure makes up the distal pressure at the downstream side of the valve. The threshold pressure can be defined by a mechanical spring preload acting upon the valve mechanism to keep it shut, which spring preload can also be adjustable and programmable by an operator. Thus, the valve mechanisms only open and allow flow when a specific proximal-distal pressure differential is achieved.
For these anti-siphoning systems to operate properly, the valve must be exposed to atmospheric pressure. This typically requires the valves to include deformable walls or apertures that are exposed to subcutaneous pressure and can translate any changes in the pressure to the valve mechanism. One of the problems with systems having deformable walls or apertures is that subcutaneous fibrotic scarring, which often develop around the shunt system once implanted, can drastically compromise the deformable wall's ability to conform and react to changes in the subcutaneous pressure. Moreover, one practical drawback with valves having deformable walls is the potential for unintentionally shutting off the valve when the patient lies on the valve and puts pressure on the deformable wall.
Accordingly, it is desirable to provide a shunt system that is effective in draining CSF in patients with hydrocephalus, while also preventing siphoning in the patient during postural changes. It is also desirable to provide such a system that is not easily compromised by subcutaneous scarring or unintentional shut off from the patient's body weight or posture. Finally, it is desirable for such a shunt system to have a low compliance biasing element acting against the valve mechanism so that the resistance of the valve to fluid flow is low.